Concentration changes of intracellular free calcium (Ca2 ions) and intracellular pH changes will be measured in molluscan giant neurons during physiological activity. Indicator dye techniques (arsenazo III and phenol red) and pH microelectrodes will be employed. Electrophysiological activity will be controlled by voltage clamp and current stimulation. Simulations of diffusional movement of Ca 2 ions and H ions will be run using parameters derived from microinjection and buffer capacity experiments run previously. The phenomenon of Ca2 ions release from bound stores by low pH will be investigated quantitatively using simultaneous electrode and dye measurements. Study of the effects of cAMP and certain neurotransmitters on internal pH will be continued.